The present invention is related to the irradiation of product packages.
Irradiation is used to treat many types of products and articles. Irradiation is used e.g. for sterilising medical devices, or for pasteurising food. Depending on the level of dose required for achieving an effect, and the type of irradiation source available, one uses gamma rays, e-beam or x-rays. These radiation types have very different properties as to penetration in matter, and the production methods are also very different.
Document xe2x80x9cElectron Beam Sterilization Technologyxe2x80x9d (Radiat. Phys. Chem. Vol. 14, pp. 403-414 James H. Bly) describes a facility for e-beam sterilization (see FIG. 6 of said document). E-beam irradiation sources are well know, easy and safe to operate and maintain. However, as shown on FIG. 1, penetration depth is limited to a value. This penetration depth depends on beam energy. Dose falls sharply beyond that depth. Penetration depths are given in g/cm2, corresponding to the actual depth multiplied by product density. The optimum thickness Ropt is the depth at which the dose is equal to the dose at the surface where the electron beam enters. This Ropt is the maximum allowable thickness of product for reaching at least a specified dose across full depth. As shown on FIG. 1, this optimum thickness is about 3.9 g/cm2 for a 10 MeV electron beam, or 3.9 cm for a product of unit density. For a product having a density of 0.5 the optimum thickness would be 7.8 cm. As an alternative, a thicker product may be treated by double-sided irradiation (FIG. 2). Product packages may be characterised by their effective thickness, defined as density multiplied by thickness for a homogenous material. For a non-homogenous material, the effective thickness is defined as the integral of the local density along the beam trajectory, and is varying along the surface of the product package. Effective thicknesses are also expressed in g/cm2.
As shown on FIG. 3, X-ray or gamma rays have a much better ability to penetrate matter. Gamma rays are produced by a radioactive isotope. A widely used isotope for such purpose is Cobalt 60. X-ray can be produced by impinging an electron beam on a conversion window made of a high Z material. In this process, a large part of the beam energy (above 90%) is lost and must be evacuated as waste heat. Therefore, the e-beam irradiation is the most efficient and economic process, provided the thickness and density of the product are not above the penetration depth. In most applications, the sizes and density of the product is known in advance, and a choice of the irradiation source is made once and for all. In the treatment of mail against biological threats, however, some letters or letter packages might contain higher density parts, including metallic pieces. In that case, irradiation by e-beam would not be effective. Depending on the required level of bacteria elimination, the required doses might be from 10 kGy to 56 kGy. These values are very high, and the treatment by X-rays, would appear to be impractical because it requires many hours for processing one package.
The treatment of mail against biological threats (e.g. anthrax) differs from other biological irradiation treatments, in that contaminants are easily made air-borne. This means that in any treatment requiring more that one pass through the beam, the possibility of recontamination or cross-contamination (from partially or non-irradiated product) is possible.
Installation for processing products by e-beam are known e.g. from the above cited document. An installation for processing either by e-beam or by X-rays is known from U.S. Pat. No. 4,484,341. In this installation, the products are recirculated in a closed path, thereby increasing the risk of cross-contamination.
Moreover, irradiation by e-beam and by X-ray is performed using the same packaging size. If that size is chosen as the optimal size for penetration by an e-beam, the same size will be far from optimal for X-ray, most of the energy traversing the product and being wasted in the background. If the size is chosen as the optimal size for X-ray, the e-beam will not penetrate the full product depth.
As an alternative to sterilisation by irradiation, it is known to sterilise products by submitting said products to a sterilant gas. Ethylene oxide or chlorine dioxide are gasses used in such methods. An example of such a method is given in U.S. Pat. No. 4,770,851. These methods have the drawback that they need a long processing time, including a pre-processing and post-processing. Moreover the gases that are used are highly toxic and dangerous.
Therefore none of these installations have the capability of treating products of different densities with acceptable throughput, and under the right security conditions.
The present invention aims to provide an irradiation apparatus and method allowing to treat a flow of product packages having a density unknown in advance, with the capability to achieve a minimal dose, without impairing throughput and processing time, and minimising risks of contaminating treated products by non-treated or partially treated products. These goals are particularly critical for treating mail against potential biological threats, such as anthrax.
The present invention is related to a method for irradiating product packages, comprising the steps of:
pre-defining a threshold of maximum effective dimension;
measuring the maximum effective dimension of a product package;
comparing said maximum effective dimension to the threshold;
directing said product package either into a first processing unit or into a second processing unit for sterilising said product package, depending on the effective dimension of the product package, said product package being directed into the first processing unit wherein the product package is sterilised with an electron-beam if its maximum effective dimension is everywhere under the threshold, or said product package being directed into the second processing unit wherein the product package is sterilised with alternative sterilization means, if the maximum effective dimension of the product package has at least an area with an effective dimension above the threshold.
It is meant by effective dimension the effective thickness measured along the direction of the electron-beam.
According to one preferred embodiment, the alternative sterilisation means are a gas sterilisation process.
According to one preferred embodiment, the alternative sterilisation means are an X-ray or gamma irradiation process.
Preferably, product packages having at least an area with an effective dimension above said threshold are grouped in arrays, a number of arrays are stacked, and said stack is irradiated from the side by an X-ray or gamma source. The stack is rotated in front of the X-ray or gamma source during irradiation, thereby improving the uniformity of the dose throughout the volume.
Preferably, sterilising a package with an e-beam in the first processing unit source is performed by an e-beam directed along the shortest dimension of the trays, i.e. from above or from below, for trays lying flat on a conveyor system.
The invention also pertains to an apparatus for irradiating product packages, comprising an irradiation source, a shielding, comprising an entry maze, an exit maze, and a conveyor device, a detection device for detecting product packages having everywhere an effective dimension below some threshold, means for directing said product packages to an e-beam source, means for directing other product packages to alternative means. Said alternative means may be a gas sterilant device or an X-ray or gamma irradiation device.
The apparatus may comprises means for grouping packages having at least an area with an effective dimension above said threshold in arrays, for stacking a number of said arrays, and for irradiating said stack from the side by an X-ray or gamma source. The apparatus may comprise a turntable for rotating said stack during irradiation.
The threshold is preferably comprised between 1 and 6 g/cm2, depending on the beam energy.
The present invention is also related to the use of an irradiation apparatus for creating both an e-beam in a first processing unit and an X-ray beam or gamma-beam in a second processing unit and for irradiating product packages with either said electron-beam or said X-ray beam or gamma-beam at least depending upon whether the maximum effective dimension of said product package is below or above a predefined threshold, respectively.